Why do Duke AI destroy their primary title?

In my Ironman game, it happened very often that after I granted a Duchy to a Count, they destroyed it after a while, despite they didn't have any other Duchy.

Why would they do it, and why so often?

And how? I was under the impression that you couldn't destroy your primary title.

• Note: this might have often been not the de-jure Duchy of their County, but still I wouldn't see why nor how.
• Remark: we're not talking about characters with too many Duchies so that they have a penalty, we're talking about characters with just one Duchy, destroying that.

Solution 1:

If you don't control any De Jure counties in a Duchy, (either directly or via your vassals) the title will automatically be dissolved for AI characters after a few months grace period. (I believe Player Characters get either more of a grace period, or at least an Outliner notification).

As you've correctly deduced, this is not normally an option, as it is impossible to destroy your primary title.

In order to prevent this from happening in the future, make sure that any duchies you dole out include de jure counties.